I updated to the latest launcher, started my 13w21a profile and loaded my single player world. Then it interrupted the loading and returned to world selection for no appearent reason. I tried loading the world again.
What I expected to happen was...:
The world would load normally.
What actually happened was...:
The game crashed because the save file is "being accessed from another location".
Steps to Reproduce:
1. Load a single player world. I'm not sure how to reproduce the interruption, it has only happened once so far.
2. If it was interrupted, load it again.